Thrombin-mediated phosphoinositide hydrolysis in Chinese hamster ovary cells overexpressing phospholipase C-delta 1

Negative self-regulation of transient receptor potential canonical 4 by the specific interaction with phospholipase C-δ1

Transient receptor potential canonical (TRPC) channels are non-selective calcium-permeable cation channels. It is suggested that TRPC4β is regulated by phospholipase C (PLC) signaling and is especially maintained by phosphatidylinositol 4,5-bisphosphate (PIP₂). In this study, we present the regulation mechanism of the TRPC4 channel with PIP₂ hydrolysis which is mediated by a channel-bound PLCδ1 but not by the G_qPCR signaling pathway. Our electrophysiological recordings demonstrate that the Ca²⁺ via an open TRPC4 channel activates PLCδ1 in the physiological range, and it causes the decrease of current amplitude. The existence of PLCδ1 accelerated PIP₂ depletion when the channel was activated by an agonist. Interestingly, PLCδ1 mutants which have lost the ability to regulate PIP₂ level failed to reduce the TRPC4 current amplitude. Our results demonstrate that TRPC4 self-regulates its activity by allowing Ca²⁺ ions into the cell and promoting the PIP₂ hydrolyzing activity of PLCδ1.

Differential Regulation of Multiple Steps in Inositol 1,4,5-Trisphosphate Signaling by Protein Kinase C Shapes Hormone-stimulated Ca2+ Oscillations

How Ca(2+) oscillations are generated and fine-tuned to yield versatile downstream responses remains to be elucidated. In hepatocytes, G protein-coupled receptor-linked Ca(2+) oscillations report signal strength via frequency, whereas Ca(2+) spike amplitude and wave velocity remain constant. IP3 uncaging also triggers oscillatory Ca(2+) release, but, in contrast to hormones, Ca(2+) spike amplitude, width, and wave velocity were dependent on [IP3] and were not perturbed by phospholipase C (PLC) inhibition. These data indicate that oscillations elicited by IP3 uncaging are driven by the biphasic regulation of the IP3 receptor by Ca(2+), and, unlike hormone-dependent responses, do not require PLC. Removal of extracellular Ca(2+) did not perturb Ca(2+) oscillations elicited by IP3 uncaging, indicating that reloading of endoplasmic reticulum stores via plasma membrane Ca(2+) influx does not entrain the signal. Activation and inhibition of PKC attenuated hormone-induced Ca(2+) oscillations but had no effect on Ca(2+) increases induced by uncaging IP3. Importantly, PKC activation and inhibition differentially affected Ca(2+) spike frequencies and kinetics. PKC activation amplifies negative feedback loops at the level of G protein-coupled receptor PLC activity and/or IP3 metabolism to attenuate IP3 levels and suppress the generation of Ca(2+) oscillations. Inhibition of PKC relieves negative feedback regulation of IP3 accumulation and, thereby, shifts Ca(2+) oscillations toward sustained responses or dramatically prolonged spikes. PKC down-regulation attenuates phenylephrine-induced Ca(2+) wave velocity, whereas responses to IP3 uncaging are enhanced. The ability to assess Ca(2+) responses in the absence of PLC activity indicates that IP3 receptor modulation by PKC regulates Ca(2+) release and wave velocity.

Leishmania donovani infection enhances lateral mobility of macrophage membrane protein which is reversed by liposomal cholesterol

Background

The protozoan parasite Leishmania donovani (LD) reduces cellular cholesterol of the host possibly for its own benefit. Cholesterol is mostly present in the specialized compartment of the plasma membrane. The relation between mobility of membrane proteins and cholesterol depletion from membrane continues to be an important issue. The notion that leishmania infection alters the mobility of membrane proteins stems from our previous study where we showed that the distance between subunits of IFNγ receptor (R1 and R2) on the cell surface of LD infected cell is increased, but is restored to normal by liposomal cholesterol treatment.

Methodology/Principal Findings

We determined the lateral mobility of a membrane protein in normal, LD infected and liposome treated LD infected cells using GFP-tagged PLCδ1 as a probe. The mobility of PLCδ1 was computationally analyzed from the time lapse experiment using boundary distance plot and radial profile movement. Our results showed that the lateral mobility of the membrane protein, which is increased in infection, is restored to normal upon liposomal cholesterol treatment. The results of FRAP experiment lent further credence to the above notion. The membrane proteins are intimately linked with cellular actin and alteration of cellular actin may influence lateral mobility. We found that F-actin is decreased in infection but is restored to normal upon liposomal cholesterol treatment as evident from phalloidin staining and also from biochemical analysis by immunoblotting.

Conclusions/Significances

To our knowledge this is the first direct demonstration that LD parasites during their intracellular life cycle increases lateral mobility of membrane proteins and decreases F-actin level in infected macrophages. Such defects may contribute to ineffective intracellular signaling and other cellular functions.

The protozoan parasites, Leishmania donovani, replicate within the macrophages of the mammalian hosts. During its intracellular lifecycle, the parasite induces a wide variety of defects in the membrane homeostasis. Membrane bound receptor molecules are important for interacting with external stimuli. Our study very clearly showed that there is an increase in the mobility of membrane protein coupled with decrease in F-actin in infected cells, which may be corrected by liposomal cholesterol treatment. This observation indicates that intracellular parasite may alter the membrane biology of infected cells which may dampen overall cellular function.

Imaging of Ca2+ and related signaling molecules and investigation of their functions in the brain

Intracellular Ca(2+) signaling, and related mechanisms involving inositol 1,4,5-trisphosphate (IP(3)), nitric oxide, and the excitatory neurotransmitter glutamate, play a major role in the regulation of cellular function in the brain. Due to the complex morphology of central neurons, the correct spatiotemporal distribution of signaling molecules is essential. Thus, imaging studies have been particularly useful in elucidating the functions of these signaling molecules. The advancement of imaging methods, together with the development of a new method for the specific inhibition of intracellular IP(3) signaling, have made it possible to identify pathways that are regulated by Ca(2+) signals in the brain, including Ca(2+)-dependent synaptic maintenance and glial cell-dependent neurite growth. Further investigation of Ca(2+)-related signaling is expected to increase our understanding of brain function in the future.

Phospholipase C-delta1 modulates sustained contraction of rat mesenteric small arteries in response to noradrenaline, but not endothelin-1

Vasoconstrictors activate phospholipase C (PLC), which hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP(2)), leading to calcium mobilization, protein kinase C activation, and contraction. Our aim was to investigate whether PLC-delta(1), a PLC isoform implicated in alpha(1)-adrenoreceptor signaling and the pathogenesis of hypertension, is involved in noradrenaline (NA) or endothelin (ET-1)-induced PIP(2) hydrolysis and contraction. Rat mesenteric small arteries were studied. Contractility was measured by pressure myography, phospholipids or inositol phosphates were measured by radiolabeling with (33)Pi or myo-[(3)H]inositol, and caveolae/rafts were prepared by discontinuous sucrose density centrifugation. PLC-delta(1) was localized by immunoblot analysis and neutralized by delivery of PLC-delta(1) antibody. The PLC inhibitor U73122, but not the negative control U-73342, markedly inhibited NA and ET-1 contraction but had no effect on potassium or phorbol ester contraction, implicating PLC activity in receptor-mediated smooth muscle contraction. PLC-delta(1) was present in caveolae/rafts, and NA, but not ET-1, stimulated a rapid twofold increase in PLC-delta(1) levels in these domains. PLC-delta(1) is calcium dependent, and removal of extracellular calcium prevented its association with caveolae/rafts in response to NA, concomitantly reducing NA-induced [(33)P]PIP(2) hydrolysis and [(3)H]inositol phosphate formation but with no effect on ET-1-induced [(33)P]PIP(2) hydrolysis. Neutralization of PLC-delta(1) by PLC-delta(1) antibody prevented its caveolae/raft association and attenuated the sustained contractile response to NA compared with control antibodies. In contrast, ET-1-induced contraction was not affected by PLC-delta(1) antibody. These results indicate the novel and selective role of caveolae/raft localized PLC-delta(1) in NA-induced PIP(2) hydrolysis and sustained contraction in intact vascular tissue.

G-protein-coupled receptor agonists activate endogenous phospholipase Cepsilon and phospholipase Cbeta3 in a temporally distinct manner

Phospholipase Cepsilon (PLCepsilon) is one of the newest members of the phosphatidylinositol-specific phospholipase C (PLC) family. Previous studies have suggested that G-protein-coupled receptors (GPCRs) stimulate phosphoinositide (PI) hydrolysis by activating PLCbeta isoforms through G(q) family G proteins and Gbetagamma subunits. Using RNA interference to knock down PLC isoforms, we demonstrate that the GPCR agonists endothelin (ET-1), lysophosphatidic acid (LPA), and thrombin, acting through endogenous receptors, couple to both endogenous PLCepsilon and the PLCbeta isoform, PLCbeta3, in Rat-1 fibroblasts. Examination of the temporal activation of these PLC isoforms, however, reveals agonist- and isoform-specific profiles. PLCbeta3 is activated acutely within the first minute of ET-1, LPA, or thrombin stimulation but does not contribute to sustained PI hydrolysis induced by LPA or thrombin and accounts for only part of ET-1 sustained stimulation. PLCepsilon, on the other hand, predominantly accounts for sustained PI hydrolysis. Consistent with this observation, reconstitution of PLCepsilon in knockdown cells dose-dependently increases sustained, but not acute, agonist-stimulated PI hydrolysis. Furthermore, combined knockdown of both PLCepsilon and PLCbeta3 additively inhibits PI hydrolysis, suggesting independent regulation of each isoform. Importantly, ubiquitination of inositol 1,4,5-trisphosphate receptors correlates with sustained, but not acute, activation of PLCepsilon or PLCbeta3. In conclusion, GPCR agonists ET-1, LPA, and thrombin activate endogenous PLCepsilon and PLCbeta3 in Rat-1 fibroblasts. Activation of these PLC isoforms displays agonist-specific temporal profiles; however, PLCbeta3 is predominantly involved in acute and PLCepsilon in sustained PI hydrolysis.

Phospholipase Cβ2 Binds to and Inhibits Phospholipase Cδ1

Phospholipase Cbeta (PLCbeta) isoforms, which are under the control of Galphaq and Gbetagamma subunits, generate Ca2+ signals induced by a broad array of extracellular agonists, whereas PLCdelta isoforms depend on a rise in cytosolic Ca2+ for their activation. Here we find that PLCbeta2 binds strongly to PLCdelta1 and inhibits its catalytic activity in vitro and in living cells. In vitro, this PLC complex can be disrupted by increasing concentrations of free Gbetagamma subunits. Such competition has consequences for signaling, because in HEK293 cells PLCbeta2 suppresses elevated basal [Ca2+] and inositol phosphates levels and the sustained agonist-induced elevation of Ca2+ levels caused by PLCdelta1. Also, expression of both PLCs results in a synergistic release of [Ca2+] upon stimulation in A10 cells. These results support a model in which PLCbeta2 suppresses the basal catalytic activity of PLCdelta1, which is relieved by binding of Gbetagamma subunits to PLCbeta2 allowing for amplified calcium signals.

Activation of PLC-delta1 by Gi/o-coupled receptor agonists

The mechanism of phospholipase (PLC)-delta activation by G protein-coupled receptor agonists was examined in rabbit gastric smooth muscle. Ca(2+) stimulated an eightfold increase in PLC-delta1 activity in permeabilized muscle cells. Treatment of dispersed or cultured muscle cells with three G(i/o)-coupled receptor agonists (somatostatin, delta-opioid agonist [D-Pen(2),D-Pen(5)]enkephalin, and A(1) agonist cyclopentyl adenosine) caused delayed increase in phosphoinositide (PI) hydrolysis (8- to 10-fold) that was strongly inhibited by overexpression of dominant-negative PLC-delta1(E341R/D343R; 65-76%) or constitutively active RhoA(G14V). The response coincided with capacitative Ca(2+) influx and was not observed in the absence of extracellular Ca(2+), but was partly inhibited by nifedipine (16-30%) and strongly inhibited by SKF-96365, a blocker of store-operated Ca(2+) channels. Treatment of the cells with a G(q/13)-coupled receptor agonist, CCK-8, caused only transient, PLC-beta1-mediated PI hydrolysis. Unlike G(i/o)-coupled receptor agonists, CCK-8 activated RhoA and stimulated RhoA:PLC-delta1 association. Inhibition of RhoA activity with C3 exoenzyme or by overexpression of dominant-negative RhoA(T19N) or Galpha(13) minigene unmasked a delayed increase in PI hydrolysis that was strongly inhibited by coexpression of PLC-delta1(E341R/D343R) or by SKF-96365. Agonist-independent capacitative Ca(2+) influx induced by thapsigargin stimulated PI hydrolysis (8-fold), which was partly inhibited by nifedipine ( approximately 25%) and strongly inhibited by SKF-96365 ( approximately 75%) and in cells expressing PLC-delta1(E341R/D343R). Agonist-independent Ca(2+) release or Ca(2+) influx via voltage-gated Ca(2+) channels stimulated only moderate PI hydrolysis (2- to 3-fold), which was abolished by PLC-delta1 antibody or nifedipine. We conclude that PLC-delta1 is activated by G(i/o)-coupled receptor agonists that do not activate RhoA. The activation is preferentially mediated by Ca(2+) influx via store-operated Ca(2+) channels.

Cortically restricted production of IP3 leads to propagation of the fertilization Ca2+ wave along the cell surface in a model of the Xenopus egg

The fertilization Ca2+ wave in Xenopus laevis is a single, large wave of elevated free cytosolic Ca2+ concentration that emanates from the point of sperm-egg fusion and traverses the entire diameter of the egg. This phenomenon appears to involve an increase in inositol-1,4,5-trisphosphate (IP3) resulting from interaction of the sperm and egg, which then results in the activation of the endoplasmic reticulum Ca2+ release machinery. We have proposed models based on a static elevated distribution of IP3, and dynamic [IP3], however, these models have suggested that the fertilization wave passes through the center of the egg. Complementing these earlier models, we propose a more detailed model of the fertilization Ca2+ wave in Xenopus eggs to explore the hypothesis that IP3 is produced only at or near the plasma membrane. In this case, we find that the wave propagates primarily through the cortex of the egg, and that Ca2+ -induced production of IP3 at the plasma membrane allows IP3 to propagate in advance of the wave. Our model includes Ca2+ -dependent production of IP3 at the plasma membrane and IP3 degradation. Simulations in 1 dimension and axi-symmetric 3 dimensions illustrate the basic features of the wave.

A wave of IP3 production accompanies the fertilization Ca2+ wave in the egg of the frog, Xenopus laevis: theoretical and experimental support

The fertilization Ca2+ wave in Xenopus laevis is a single, large wave of elevated free Ca2+ that is initiated at the point of sperm-egg fusion and traverses the entire width of the egg. This Ca2+ wave involves an increase in inositol-1,4,5-trisphosphate (IP3) resulting from the interaction of the sperm and egg, which then results in the activation of the endoplasmic reticulum Ca2+ release machinery. The extraordinarily large size of this cell (1.2 mm diameter) together with the small surface region of sperm-receptor activation makes special demands on the IP3-dependent Ca2+ mobilizing machinery. We propose a detailed model of the fertilization Ca2+ wave in Xenopus eggs that requires an accompanying wave of IP3 production. While the Ca2+ wave is initiated by a localized increase of IP3 near the site of sperm-egg fusion, the Ca2+ wave propagates via IP3 production correlated with the Ca2+ wave-possibly via Ca(2+)-mediated PLC activation. Such a Ca(2+)-mediated IP(3) production wave has not been required previously to explain the fertilization Ca2+ wave in eggs; we argue this is necessary to explain the observed IP3 dynamics in Xenopus eggs. To test our hypothesis, we have measured the IP3 levels from 20 nl "sips" of the egg cortex during wave propagation. We were unable to detect the low IP3 levels in unfertilized eggs, but after fertilization, [IP3] ranged from 175 to 430 nM at the sperm entry point and from 120 to 700 nM 90 degrees away once the Ca2+ wave passed that region about 2 min after fertilization. Prior to the Ca2+ wave reaching that region the IP3 levels were undetectable. Since significant IP3 could not diffuse to this region from the sperm entry point within 2 min, this observation is consistent with a regenerative wave of IP3 production.

Reactive oxygen species as mediators of signal transduction in ischemic preconditioning

Ischemic preconditioning (IPC) is a most powerful endogenous mechanism for myocardial protection against ischemia/reperfusion injury. It is now apparent that reactive oxygen species (ROS) generated in the mitochondrial respiratory chain act as a trigger of IPC. ROS mediate signal transduction in the early phase of IPC through the posttranslational modification of redox-sensitive proteins. ROS-mediated activation of Src tyrosine kinases serves a scaffold for interaction of proteins recruited by G protein-coupled receptors and growth factor receptors that is necessary for amplification of cardioprotective signal transduction. Protein kinase C (PKC) plays a central role in this signaling cascade. A crucial target of PKC is the mitochondrial ATP-sensitive potassium channel, which acts as a trigger and a mediator of IPC. Mitogen-activated protein (MAP) kinases (extracellular signal-regulated kinase, p38 MAP kinase, and c-Jun NH(2)-terminal kinase) are thought to exist downstream of the Src-PKC signaling module, although the role of MAP kinases in IPC remains undetermined. The late phase of IPC is mediated by cardioprotective gene expression. This mechanism involves redox-sensitive activation of transcription factors through PKC and tyrosine kinase signal transduction pathways that are in common with the early phase of IPC. The effector proteins then act against myocardial necrosis and stunning presumably through alleviation of oxidative stress and Ca(2+) overload. Elucidation of IPC-mediated complex signaling processes will help in the development of more effective pharmacological approaches for prevention of myocardial ischemia/reperfusion injury.

Visualization of IP(3) dynamics reveals a novel AMPA receptor-triggered IP(3) production pathway mediated by voltage-dependent Ca(2+) influx in Purkinje cells

IP(3) signaling in Purkinje cells is involved in the regulation of cell functions including LTD. We have used a GFP-tagged pleckstrin homology domain to visualize IP(3) dynamics in Purkinje cells. Surprisingly, IP(3) production was observed in response not only to mGluR activation, but also to AMPA receptor activation in Purkinje cells in culture. AMPA-induced IP(3) production was mediated by depolarization-induced Ca(2+) influx because it was mimicked by depolarization and was blocked by inhibition of the P-type Ca(2+) channel. Furthermore, trains of complex spikes, elicited by climbing fiber stimulation (1 Hz), induced IP(3) production in Purkinje cells in cerebellar slices. These results revealed a novel IP(3) signaling pathway in Purkinje cells that can be elicited by synaptic inputs from climbing fibers.

Phospholipase C-gamma mediates the hydrolysis of phosphatidylinositol, but not of phosphatidylinositol 4,5-bisphoshate, in carbamylcholine-stimulated islets of langerhans

In pancreatic islets the activation of phospholipase C (PLC) by the muscarinic receptor agonist carbamyolcholine (carbachol) results in the hydrolysis of both phosphatidylinositol 4,5-bisphosphate (PtdInsP(2)) and phosphatidylinositol (PtdIns). Here we tested the hypothesis that PtdIns hydrolysis is mediated by PLCgamma1, which is known to be regulated by activation of tyrosine kinases and PtdIns 3-kinase. PtdIns breakdown was more sensitive than that of PtdInsP(2) to the tyrosine kinase inhibitor, genistein. Conversely, the tyrosine phosphatase inhibitor, vanadate, alone promoted PtdIns hydrolysis and acted non-additively with carbachol. Vanadate did not stimulate PtdInsP(2) breakdown. Carbachol also stimulated a rapid (maximal at 1-2 min) tyrosine phosphorylation of several islet proteins, although not of PLCgamma1 itself. Two structurally unrelated inhibitors of PtdIns 3-kinase, wortmannin and LY294002, more effectively attenuated the hyrolysis of PtdIns compared with PtdInsP(2). Adenovirally mediated overexpression of PLCgamma1 significantly increased carbachol-stimulated PtdIns hydrolysis without affecting that of PtdInsP(2). Conversely overexpression of PLCbeta1 up-regulated the PtdInsP(2), but not PtdIns, response. These results indicate that the hydrolysis of PtdIns and PtdInsP(2) are independently regulated in pancreatic islets and that PLCgamma1 selectively mediates the breakdown of PtdIns. The activation mechanism of PLCgamma involves tyrosine phosphorylation (but not of PLCgamma directly) and PtdIns 3-kinase. Our findings point to a novel bifurcation of signaling pathways downstream of muscarinic receptors and suggest that hydrolysis of PtdIns and PtdInsP(2) might serve different physiological ends.

Fuel and hormone regulation of phospholipase C beta 1 and delta 1 overexpressed in RINm5F pancreatic beta cells

The mechanism by which glucose and other fuels stimulate phosphoinositide-specific phospholipase C (PLC) in pancreatic islet beta cells is not known. Previous studies have suggested that glucose may couple to PLC beta 1 and PLC delta 1. To determine directly if fuels activate these PLC isozymes, clones stably overexpressing PLC beta 1 or PLC delta 1 were generated in the fuel-sensitive beta cell line RINm5F, and secretagogue regulation of these PLC isoforms was determined. Overexpression of PLC beta 1 or PLC delta 1 significantly increased PLC activity in isolated cell fractions, consistent with overexpression of active PLC isoforms in these clones. In paired experiments, stimulation of inositol phosphate (IP) accumulation by the fuel glyceraldehyde was enhanced in clones overexpressing PLC beta 1, in parallel with the G-protein alpha subunit activator, AlF(4)(-), suggesting a coupling between glyceraldehyde and this PLC isoform. In contrast, overexpression of PLC delta 1 had no effect on glyceraldehyde- or AlF(4)(-)-stimulated IP accumulation. Similarly, IP accumulation stimulated by ionomycin was enhanced in PLC beta 1, but not PLC delta 1 clones, indicating that increases in intracellular free calcium [Ca(2+)](i) can regulate PLC beta 1 but not PLC delta 1 overexpressed in this cell line. Interestingly, [Arg(8)] vasopressin-stimulated, but not carbachol-stimulated, IP accumulation was significantly increased in clones overexpressing either PLC beta 1 or PLC delta 1. These studies illustrate unique pathways coupling diverse secretagogues to specific PLC isoforms in islet beta cells, and demonstrate that glyceraldehyde can activate PLC beta 1 but not PLC delta 1; whereas, vasopressin, but not carbachol, can stimulate either isoform.

Structure, function, and control of phosphoinositide-specific phospholipase C

Phosphoinositide-specific phospholipase C (PLC) subtypes beta, gamma, and delta comprise a related group of multidomain phosphodiesterases that cleave the polar head groups from inositol lipids. Activated by all classes of cell surface receptor, these enzymes generate the ubiquitous second messengers inositol 1,4, 5-trisphosphate and diacylglycerol. The last 5 years have seen remarkable advances in our understanding of the molecular and biological facets of PLCs. New insights into their multidomain arrangement and catalytic mechanism have been gained from crystallographic studies of PLC-delta(1), while new modes of controlling PLC activity have been uncovered in cellular studies. Most notable is the realization that PLC-beta, -gamma, and -delta isoforms act in concert, each contributing to a specific aspect of the cellular response. Clues to their true biological roles were also obtained. Long assumed to function broadly in calcium-regulated processes, genetic studies in yeast, slime molds, plants, flies, and mammals point to specific and conditional roles for each PLC isoform in cell signaling and development. In this review we consider each subtype of PLC in organisms ranging from yeast to mammals and discuss their molecular regulation and biological function.

Involvement of EF hand motifs in the Ca(2+)-dependent binding of the pleckstrin homology domain to phosphoinositides

The pleckstrin homology (PH) domains of phospholipase C (PLC)-delta1 and a related catalytically inactive protein, p130, both bind inositol phosphates and inositol lipids. The binding to phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] by PLC-delta1 is proposed to be the critical interaction required for membrane localization to where the substrate resides; it is also required for the Ca(2+)-dependent activation of PLC-delta1 observed in the permeabilized cells. In the proximity of the PH domain, both PLC-delta1 and p130 possess the EF-hand domain, containing classical motifs implicated in calcium binding. Therefore, in the present study we examined whether the binding of the PH domain to PtdIns(4,5)P2 is regulated by changes in free Ca2+ concentration within the physiological range. A Ca2+ dependent increase in the binding to PtdIns(4,5)P2 was observed with a full-length PLC-delta1, while the isolated PH domain did not show any Ca2+ dependence. However, the connection of the EF-hand motifs to the PH domain restored the Ca2+ dependent increase in binding, even in the absence of the C2 domain. The p130 protein showed similar properties to PLC-delta1, and the EF-hand motifs were again required for the PH domain to exhibit a Ca2+ dependent increase in the binding to PtdIns(4,5)P2. The isolated PH domains from several other proteins which have been demonstrated to bind PtdIns(4,5)P2 showed no Ca2+ dependent enhancement of binding. However, when present within a chimera also containing PLC-delta1 EF-hand motifs, the Ca2+ dependent binding was again observed. These results suggest that the binding of Ca2+ to the EF-hand motifs can modulate binding to PtdIns(4,5)P2 mediated by the PH domain.

Phospholipase C-delta1 is activated by capacitative calcium entry that follows phospholipase C-beta activation upon bradykinin stimulation

To characterize the regulatory mechanism of phospholipase C-delta1 (PLC-delta1) in the bradykinin (BK) receptor-mediated signaling pathway, we used a clone of PC12 cells, which stably overexpress PLC-delta1 (PC12-D1). Stimulation with BK induced a significantly higher Ca(2+) elevation and inositol 1,4,5-trisphosphate (IP(3)) production with a much lower half-maximal effective concentration (EC(50)) of BK in PC12-D1 cells than in wild type (PC12-W) or vector-transfected (PC12-V) cells. However, BK-induced intracellular Ca(2+) release and IP(3) generation was similar between PC12-V and PC12-D1 cells in the absence of extracellular Ca(2+), suggesting that the availability of extracellular Ca(2+) is essential to the activation of PLC-delta1. When PC12-D1 cells were treated with agents that induce Ca(2+) influx, more IP(3) was produced, suggesting that the Ca(2+) entry induces IP(3) production in PC12-D1 cells. Furthermore, the additional IP(3) production after BK-induced capacitative calcium entry was detected in PC12-D1 cells, suggesting that PLC-delta1 is mainly activated by capacitative calcium entry. When cells were stimulated with BK in the presence of extracellular Ca(2+), [(3)H]norepinephrine secretion was much greater from PC12-D1 cells than from PC12-V cells. Our results suggest that PLC-delta1 is activated by capacitative calcium entry following the activation of PLC-beta, additively inducing IP(3) production and Ca(2+) rise in BK-stimulated PC12 cells.

Molecular cloning and expression analysis of a mouse phospholipase C-delta1

We describe here the molecular cloning and expression analysis of mouse PLC-delta1 (mPLC-delta1), a key enzyme in cell signal transduction. A mouse brain cDNA library was screened in order to isolate the mPLC-delta1 cDNA. The mPLC-delta1 cDNA was 2660 bp in length. The predicted open reading frame encodes a protein of 756 amino acids with an estimated molecular mass of 85 kDa. The deduced amino acid sequence exhibits 96.9% and 92.7% identity with the sequence of rat and human PLC-delta1, respectively. The mPLC-delta1 mRNA was highly expressed in brain, heart, lung, and testis. We found that transcripts of mPLC-delta1 are present in almost all regions of mouse brain examined, implying that the enzyme may play a role in some fundamental cellular process in brain. In male reproductive tract, mPLC-delta1 mRNA was widely expressed in the epididymis as well as in the testis. In situ hybridization studies indicate that distribution of mPLC-delta1 mRNA in mouse testis is discrete and unique. The expression of mPLC-delta1 mRNA was defined in the periphery of each seminiferous tubule, especially in spermatogonia, which might imply that mPLC-delta1 plays a role in proliferation of spermatogonia. To the best our knowledge, this is the first report to demonstrate the high expression of mPLC-delta1 mRNA in spermatogonia of testis. Taken together, these results suggest that mPLC-delta1 may carry out fundamental roles in almost all of mouse tissues, especially in brain and specific roles in testis.

Families of phosphoinositide-specific phospholipase C: structure and function

A large number of extracellular signals stimulate hydrolysis of phosphatidylinositol 4,5-bisphosphate by phosphoinositide-specific phospholipase C (PI-PLC). PI-PLC isozymes have been found in a broad spectrum of organisms and although they have common catalytic properties, their regulation involves different signalling pathways. A number of recent studies provided an insight into domain organisation of PI-PLC isozymes and contributed towards better understanding of the structural basis for catalysis, cellular localisation and molecular changes that could underlie the process of their activation.

Changes in the expression of protein kinase C (PKC), phospholipases C (PLC) and D (PLD) isoforms in spleen, brain and kidney of the aged rat: RT-PCR and Western blot analysis

The age-dependent changes of expression of protein kinase C (PKC), phospholipase C (PLC) and phospholipase D (PLD) isozymes were analyzed in spleen, brain and kidney of young-adult (12-16 week-old) and aged (82-88 week-old) rats. The activities of spleen cPKC and nPKC were significantly decreased by nearly 35 and 30% in aged rats compared to those of young adults, respectively (P < 0.05). The level of PKC beta1 was significantly decreased in aged rats as assessed by RT-PCR and Western blot analyses. In aged rat brain where the activity of cPKC was significantly decreased by nearly 25% (P < 0.05), PKC alpha and beta1 isozymes were significantly down-regulated. In kidney, the level of PKC beta2 was decreased. In spleen the both mRNA and protein levels of PLC beta2 and gamma2 were significantly down-regulated in aged rat (P < 0.05). PLC beta1 was also significantly lower in aged rat brain (P < 0.05) as assessed by RT-PCR and Western blotting. Moreover, PLC beta1 was significantly down-regulated in both mRNA and protein levels in aged rat kidney (P < 0.05). In contrast, the tissues examined, the expressions of PLD isozymes (PLD1a, 1b and 2) were rather stable in the course of aging. These results indicate that mRNAs of PLD isozymes were rather stable but that particular PKC and PLC isozymes were down-regulated in different tissues during aging, suggesting age-dependent decline of specific PKC and PLC isozymes in organs which may, at least in part, be implicated in tissue dysfunction with aging.

Regulation of phospholipase C-delta by GTP-binding proteins-rhoA as an inhibitory modulator

The regulation of Phospholipase C (PLC)delta activity remains obscure. These studies show that PLCdelta1 activity is significantly enhanced by both guanosine thiotriphosphate (GTPgammaS) and Clostridium botulinum exoenzyme C3 (C3) but not by aluminium fluoride. C3 ADP ribosylated a 21-kDa protein in the PLCdelta1 preparation and Western blotting identified rhoA in these samples. RhoA acts as an inhibitory modulator of PLCdelta activity.

IL-13 induces tyrosine phosphorylation of phospholipase C gamma-1 following IRS-2 association in human monocytes: relationship with the inhibitory effect of IL-13 on ROI production

Here we analysed the involvement of tyrosine phosphorylation in the regulation of the initial molecular events induced by IL-13 to modulate TPA-triggered reactive oxygen intermediates (ROI) production. Our data indicate that treatment of monocytes with a protein tyrosine kinase inhibitor (herbimycin A) prevents IL-13-induced cAMP accumulation and subsequent ROI inhibition. We have previously demonstrated that cAMP accumulation depends on inositol phosphates hydrolysis (InsPs) and intracellular Ca2+ mobilisation. The inhibition of InsPs and intracellular Ca2+ release by herbimycin A suggests a primary role of tyrosine kinases upstream PLC activation. We further specify that IL-13 stimulates PLC-gamma 1 and IRS-2 tyrosine phosphorylation in human monocytes. We demonstrate for the first time that IL-13 induces the association of IRS-2 with PLC-gamma 1. We proposed here that PLC-gamma 1 is a new candidate recruited by IRS-2.

Differences in the stimulation of the phosphoinositide cycle by amine neurotransmitters in cultured rat forebrain neurones and astrocytes

In this study, we compared the stimulation by carbachol (CCh), noradrenaline (NA), and histamine (HA) of phosphoinositide hydrolysis in rat forebrain neuronal and glial cultures. When Ca2+ was omitted from the stimulation buffer (low microM extracellular Ca2+), amine-induced [3H]inositol phosphate accumulation was reduced to a higher extent in astrocytes (70-80% for CCh and NA and 100% for HA) than in neurones (around 50-60% for all the amines). Furthermore, guanosine 5'-[gamma-thio]trisphosphate (GTP[S]) stimulation of phosphoinositidase C (PIC) in membranes was 5-fold higher in neurones than in astrocytes. These results indicate differences in the mechanism of PIC stimulation in the two cell types. After 30 min stimulation in the presence of 10 mM Li+, a higher accumulation of [3H]inositol 4-monophosphate and [3H]inositol 1,4-bisphosphate than of [3H]inositol 1/3-monophosphate occurred for all agonists in neurones, whereas the opposite was observed in astrocytes. Moreover, in these cells stimulation for 5 min in the absence of Li+ produced a 2-3-fold accumulation of all metabolites of the 3-kinase pathway of inositol-1,4,5-trisphosphate metabolism but not of those of the 5-phosphatase pathway. Thus, regardless of the amine receptor stimulated, the 3-kinase route appeared to prevail in astrocytes and the 5-phosphatase pathway in neurones. The histamine response in neurones differed from that of the other agonists in that it rapidly declined. Taken together these results indicate that the heterogeneity in amine stimulation of the phosphoinositide cycle previously observed in brain slices could arise to a great extent from the cellular diversity of this preparation and be related to the differential contribution of the amine receptors located in neurones and astrocytes.

Calcium release at fertilization in starfish eggs is mediated by phospholipase Cgamma

Although inositol trisphosphate (IP3) functions in releasing Ca2+ in eggs at fertilization, it is not known how fertilization activates the phospholipase C that produces IP3. To distinguish between a role for PLCgamma, which is activated when its two src homology-2 (SH2) domains bind to an activated tyrosine kinase, and PLCbeta, which is activated by a G protein, we injected starfish eggs with a PLCgamma SH2 domain fusion protein that inhibits activation of PLCgamma. In these eggs, Ca2+ release at fertilization was delayed, or with a high concentration of protein and a low concentration of sperm, completely inhibited. The PLCgammaSH2 protein is a specific inhibitor of PLCgamma in the egg, since it did not inhibit PLCbeta activation of Ca2+ release initiated by the serotonin 2c receptor, or activation of Ca2+ release by IP3 injection. Furthermore, injection of a PLCgamma SH2 domain protein mutated at its phosphotyrosine binding site, or the SH2 domains of another protein (the phosphatase SHP2), did not inhibit Ca2+ release at fertilization. These results indicate that during fertilization of starfish eggs, activation of phospholipase Cgamma by an SH2 domain-mediated process stimulates the production of IP3 that causes intracellular Ca2+ release.

Effect of sphingomyelin and its metabolites on the activity of human recombinant PLC delta 1

In an attempt to obtain sufficient quantities of pure phospholipase C delta 1 (PLC delta 1) necessary for structural and kinetic studies, human fibroblast PLC delta 1 was cloned in the pPROEX-1 vector, expressed in E. coli cells as a (6xHis) fusion protein and purified to homogeneity. From 11 of E. coli culture 21 mg of pure PLC delta 1 was obtained by a two-step purification procedure, which includes Ni(2+)-NAT agarose and Mono S cation exchange chromatography. Catalytic properties of recombinant PLC delta 1 with respect to activation by spermine and calcium ions and inhibition by sphingomyelin were similar to or identical to PLC delta 1 purified from rat liver. Calcium activation of PLC delta 1 was dependent on the presence of spermine. Half-maximal activity was attained at 250 and 170 nM of free Ca2+ in the presence and absence of spermine, respectively. Sphingomyelin and lysosphingomyelin were mixed type inhibitors with respect to PIP2. Ceramide inhibits PLC delta 1 very weakly. GM1, which is a ceramide bound glucosidically to the oligosaccharide moiety, was a strong non-competitive inhibitor of PLC delta 1. In the absence of spermine, sphingosine and phytosphingosine weakly activated PLC delta 1. The results indicate that the effect of sphingomyelin and its metabolites on PLC delta 1 activity depends on the presence of spermine. It is postulated that, among other factors, in vivo, activity of PLC delta 1 may depend on the turnover of sphingomyelin.

Transfected adenosine A1 receptor-mediated modulation of thrombin-stimulated phospholipase C and phospholipase A2 activity in CHO cells

Thrombin receptor activation in Chinese hamster ovary (CHO) cells stimulates the hydrolysis of inositol phospholipids and the release of arachidonic acid. Our previous studies have shown that activation of the human transfected adenosine A1 receptor in CHO cells (CHO-A1) potentiates the accumulation of inositol phosphates elicited by endogenous P2U purinoceptors and CCKA receptors. In this study we have investigated whether adenosine A1 receptor activation can modulate thrombin-stimulated arachidonic acid release and/or inositol phospholipid hydrolysis in CHO-A1 cells. Thrombin stimulated [3H]arachidonic acid release and total [3H]inositol phosphate accumulation in CHO-A1 cells. Both these responses to thrombin were were insensitive to pertussis toxin. The protein kinase C activator, phorbol 12-myristate 13-acetate (PMA), potentiated thrombin-stimulated [3H]arachidonic acid. In marked contrast, PMA inhibited thrombin-stimulated [3H]inositol phosphate accumulation. The selective protein kinase C inhibitor Ro 31-8220 (3-¿1-[3-(2-isothioureido)propyl] indol-3-yl¿-4-(1-methylindol-3-yl)-3-pyrrolin-2,5-dione) had no effect on thrombin-stimulated [3H]arachidonic acid release but reversed the potentiation of thrombin-stimulated [3H]arachidonic acid release elicited by PMA. The selective adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) augmented the release of [3H]arachidonic acid produced by thrombin. Co-activation of the adenosine A1 receptor also potentiated thrombin-stimulated [3H]inositol phosphate accumulation. The synergistic interactions between the adenosine A1 receptor and thrombin were abolished in pertussis-toxin-treated cells. The potentiation of [3H]arachidonic acid release by CPA was blocked by the protein kinase C inhibitors Ro 31-8220 and GF 109203X (3-[1-[3-(dimethylamino)propyl]-1 H-indol-3-yl]-4-(1 H-indol-3-yl)- 1H-pyrrole-2,5-dione). In conclusion, thrombin receptor activation in CHO-A1 cells stimulates the accumulation of [3H]inositol phosphates and the release of [3H]arachidonic acid through pertussis-toxin-insensitive G-proteins. Experiments using PMA suggest that protein kinase C differentially regulates thrombin receptor activation of phospholipase C and phospholipase A2. Co-activation of the transfected human adenosine A1 receptor augments thrombin-stimulated phospholipase C and phospholipase A2 activity. Finally, the augmentation of phospholipase A2 activity by the adenosine A1 receptor is inhibited by selective protein kinase C inhibitors, suggesting the involvement of protein kinase C.

Structural views of phosphoinositide-specific phospholipase C: signalling the way ahead

Recent structural studies of mammalian phosphoinositide-specific phospholipase C (PI-PLC) have begun to shed light on the mechanism whereby this family of effector enzymes is able to hydrolyze phospholipid substrates to yield second messengers. PI-PLC isozymes employ a variety of modules (PH domain, EF-hand domain, SH2 domain, SH3 domain and C2 domain) that are common in proteins involved in signal transduction to reversibly interact with membranes and protein components of the signalling pathways.

Angiotensin II-stimulated phospholipase C responses of two vascular smooth muscle-derived cell lines. Role of cyclic GMP

Vascular smooth muscle cells of the spontaneously hypertensive rat (SHR) are known to show increased responsiveness to angiotensin II (Ang II) compared with cells of normotensive control Wistar-Kyoto rats (WKY). We investigated the hypothesis that differential levels of cGMP lead to the different responsiveness of the cells, using vascular smooth muscle cells in culture. cGMP levels in extracts of SHR-derived cells were lower than those of WKY-derived cells. This was true for both unstimulated cells and cells treated with equal concentrations of either sodium nitroprusside or S-nitroso-N-acetylpenicillamine. Stimulation of cells with Ang II did not affect levels of cGMP but increased levels of inositol 1, 4, 5-trisphosphate (IP3) and Ca2+, which were greater in SHR- than in WKY-derived cells. When SHR and WKY cells were preincubated with different concentrations of S-nitroso-N-acetylpenicillamine to generate similar cGMP levels in each cell type, the subsequent IP3 response to Ang II was the same in the two cell types. To reduce any influence of cGMP on responses, we permeabilized the cells with alpha-toxin. Stimulation of alpha-toxin-permeabilized the cells with high Ca2+ revealed an IP3 response in SHR- but not WKY-derived cells. Similarly, permeabilized SHR cells responded to Ang II but WKY cells did not. However, GTP and GTP gamma S elevated IP3 in both cell types. Taken together, these results indicate that the low response of WKY cells can be accounted for by the inhibitory influence of cGMP. However, when this inhibition is removed by permeabilization, further differences between the cells are revealed that will contribute to the elevated SHR response.

Evidence that phospholipase delta1 is the effector in the Gh (transglutaminase II)-mediated signaling

A new class of GTP-binding protein transglutaminase II (Gh) couples to a 69-kDa phospholipase C (PLC). An 8-amino acid region (Leu665-Lys672) of the alpha-subunit of Gh (Galphah) is involved in interaction and activation of PLC, an observation that has now been used to characterize the 69-kDa PLC further. A 20-amino acid peptide corresponding to Leu654-Leu673 of Galphah was used to prepare an affinity resin. On incubation with a partially purified PLC preparation from rat liver membranes, the affinity resin-bound approximately69- and 85-kDa proteins were recognized by an antibody to the 69-kDa PLC. Both purified 69-kDa PLC and PLC-delta1 bound to the affinity resin; moreover, antibodies to PLC-delta1 recognized the 69-kDa PLC, and antibodies to the 69-kDa PLC recognized PLC-delta1. A synthetic peptide corresponding to Leu661-Lys672 of Galphah inhibited the binding of PLC-delta1 to the affinity resin and also stimulated PLC-delta1. Reconstitution of PLC-delta1 with GTPgammaS (guanosine 5'-3-O-(thio)triphosphate)-activated Gh resulted in activation of PLC-delta1. Antibodies to Galphah also coimmunoprecipitated PLC-delta1 upon activation of Gh. These findings indicate that PLC-delta1 is the effector of Gh-mediated signaling.

Mechanisms of hormone and growth factor action in the bovine corpus luteum

The binding of hormones and growth factors to their cell surface receptors leads to an orderly cascade of events leading to activation of cytoplasmic effector molecules. The mechanism of action of luteinizing hormone involves the stimulation of multiple signal transduction effector systems including adenylyl cyclase and inositol phospholipid-specific phospholipase C (PLC). This results in the formation of second messengers that activate cAMP-dependent, Ca(2+)-dependent and lipid-dependent protein kinases. Prostaglandin F(2alpha) activates PLC which increases intracellular calcium and activates protein kinase C. This results in the activation of a series of protein kinases in the mitogen-activated protein (MAP) kinase cascade, leading to the activation of nuclear transcription factors c-fos and c-jun. Hormone responsive effector systems, therefore, operate by activating families of protein kinases which regulate cell metabolism, secretion, and gene transcription. Growth factors activate specific receptor protein tyrosine kinases which recruit additional signaling molecules (phospholipase Cgamma, phosphatidylinositol 3-kinase, Shc, Grb2, etc.) initiating a cascade of events mediated via MAP kinases. The signaling pathways activated by hormones interact or cross talk with the signaling pathways activated by growth factors. The diversity of cellular signaling mechanisms elicited by hormones and the potential for interactions with signals generated by growth factor receptor tyrosine kinases, may allow fine tuning of cellular responses during the life span of the corpus luteum.

Nitric oxide action on growth factor-elicited signals. Phosphoinositide hydrolysis and [Ca2+]i responses are negatively modulated via a cGMP-dependent protein kinase I pathway

The role of nitric oxide (NO) in the phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis and intracellular Ca2+ release responses induced by epidermal, platelet-derived, and fibroblast growth factors was investigated in three cell lines, a clone of NIH-3T3 fibroblasts overexpressing epidermal growth factor receptors and the tumoral epithelial cells A431 and KB. In all three cell types, pretreatment with NO donors decreased growth factor-induced PIP2 and Ca2+ responses, whereas pretreatment with NO synthase inhibitors increased them. The Ca2(+)-dependent PIP2 hydroysis induced by micromolar concentrations of the Ca2+ ionophore, ionomycin, was also modulated negatively and positively by NO donors and synthase inhibitors, respectively. In contrast, the Ca2+ content of the intracellular stores was unaffected by the various pretreatments employed. NO donors and synthase inhibitors induced an increase and decrease, respectively, of the intracellular cGMP formation in all three cell lines investigated. All of the effects of the NO donors were mimicked by 8-bromo-cGMP administration and abolished by pretreatment with the specific blocker of the cGMP-dependent protein kinase I, KT5823, which by itself mimicked the effects of the synthase inhibitors. Together with previous observations on G protein-coupled receptors, the present results demonstrate that PIP2 hydrolysis and Ca2+ release occur under the feedback control of NO, independently of the phospholipase C (beta, gamma, or delta type) involved and of the mechanism of activation. Such a control, which appears to be effected by the cGMP-dependent protein kinase I acting at the level of the phospholipases C themselves, might ultimately contribute to the inhibitory role of NO on growth previously observed with various cell types.

Homologous and heterologous regulation of receptor-stimulated phosphoinositide hydrolysis

Signal transduction at a diverse range of pharmacologically distinct receptors is effected by the enhanced turnover of inositol phospholipids, with the attendant formation of inositol 1,4,5-trisphosphate and diacylglycerol. Although considerable progress has been made in recent years towards the identification and characterization of the individual components of this pathway, much less is known of mechanisms that may underlie its regulation. In this review, evidence is presented for the potential regulation of inositol lipid turnover at the level of receptor, phosphoinositide-specific phospholipase C and substrate availability in response to either homologous or heterologous stimuli. Available data indicate that the extent of receptor-stimulated inositol lipid hydrolysis is regulated by multiple mechanisms that operate at different levels of the signal transduction pathway.

Reduced expression of phospholipase C-delta, a signal-transducing enzyme, in rat colon neoplasms induced by methylazoxymethanol acetate

Phospholipase C (PLC), which hydrolyzes phosphoinositides, has been implicated as a key enzyme in signal transduction. We examined the expression of an isozyme of PLC, PLC-delta, in rat colon neoplasms induced by methylazoxymethanol (MAM) acetate. Large-bowel neoplasms were observed in five of 10 rats given MAM acetate (25 mg/kg body weight, by interperitoneal injection at 6 and 7 wk of age) 40 wk after treatment. Expression of PLC-delta in the neoplasms was not detected by northern blot analysis, and a low level of expression was detected by immunoblot analysis, although PLC-delta expression was apparent in the non-neoplastic colon mucosae of MAM acetate-treated rats as well as in the colon mucosae of control rats. Furthermore, analysis by reverse transcriptase-polymerase chain reaction revealed that the ratio of the expression of PLC-delta to that of beta-actin in the neoplasms was significantly lower than the ratios in the non-neoplastic colon mucosae of carcinogen-treated and control rats (P < 0.01). However, the ornithine decarboxylase (ODC) activity in the neoplasms was significantly greater than that of the non-neoplastic and control mucosae (P < 0.001). The differences in the levels of PLC-delta expression in neoplastic and non-neoplastic tissues and the inverse correlation of PLC-delta expression with ODC activity may suggest that PLC-delta has little effect on the PLC-mediated mitogenic signaling system, at least in MAM acetate-induced colon neoplasms in rats.